1. Field of the Invention
This invention relates to a printer, especially a printer having a single print element mounted for rotation to present different character types such as letters, symbols, numbers thereon in a printing position.
2. Description of the Prior Art
In a printer of this type, the print element is of a globe-like configuration cut away in its top and has a number of characters such as letters, symbols, marks, etc. in rows and columns, for example, a total of 88 characters in 4 rows and 22 columns, or a total of 96 characters in 4 rows and 24 columns arranged thereon. Usually, upper case characters are arranged in 11 or 12 columns on one semicircular face of the globe-shaped element and lower case characters are arranged on the other semicircular face. Selection of the characters thus arranged is effected by tilting and rotating the print element to present a desired character in the printing position confronting a platen.
Various selection systems have been proposed for controlling the print element in such a single element printer. However, in such a printer, especially of the type in which the print element is mounted on a carrier travelling along a platen or a printing line, it has been proposed that all members of the selection mechanism be provided together in the carrier in order to simplify the mechanism, maintenance and adjustment etc. and to improve responsiveness.
U.S. Pat. No. 3,666,070 discloses a printer having a mechanism comprising: a differential mechanism including a pinion connected to the print element for rotation therewith, a sector gear meshing with said pinion, an axle pivotally supporting the sector gear, and a link pivoted about an axis of the pinion and having the axle fixed thereon; a first input means including a face cam and a cam follower for selectively positioning the link in any of a predetermined plurality of pivotal positions, and a spring for urging the cam follower toward the face cam; a second input means including the front cam, another cam follower and a similar spring for selectively positioning the sector gear in any of a predetermined plurality of pivotal positions relative to the axle. All the components are provided together in the carrier thereby selecting any of 11 characters included in one row on a print head, i.e., the print element.
However, the mechanism above described cannot be suitably employed for the printer requiring a printing speed as high as 16 or 17 letters/sec, which is required in an ordinary electric typewriter, which includes a print element, as abovementioned, with 22 or 24 columns of characters, because in the disclosed printer a case shifting mechanism for selecting either of the upper or the lower case characters which is necessary in conventional typewriters is not incorporated, and because when such a print element is rotated in correspondence with the high printing speed, rotational inertia of the print element may become great enough to extend the springs provided in the mechanism excessively beyond the selected angular distance of the element, which will prevent correct selection of a character column.
A German patent application No. P 20 03 158.7-27, laid open on Aug. 19, 1971 discloses an improved selection mechanism which has eliminated such defects. In this selection mechanism, a pinion connected with the print head for rotation therewith is engaged with a pivotable sector gear which is form-closedly connected with a differential lever system including a pair of pivotal levers each adapted to be form-closedly driven by two individually and cyclically drivable eccentric discs, and the case shifting operation is accomplished by following the steps of first pivoting the sector gear in a direction to bring same out of engagement with the pinion, second rotating the pinion 180 degrees through a separate means, and finally pivoting the sector gear in the other direction to reestablish appropriate engagement with the pinion. The disengagement of the sector gear from the pinion, however, causes a problem in that the print head occasionally becomes incorrectly positioned relative to a case shift key, especially where the shift key is successively depressed and released rapidly, so that the printing of a character is in the wrong case. A wrong character may be printed in response to a character key depression after a case shifting operation.
A similar problem may be considered inherent to the structure of a selection mechanism for a single element printer proposed by another U.S. Pat. No. 3,892,304, entitled "Single Print Element Print Carrier with Self-Contained Selection Function." In the printer, the selection mechanism is incorporated in a carrier and includes a multichannel barrel cam adapted to be driven through a cyclically driven key-responsive print shaft to laterally translate a selected length, and a dual rack member connected with the barrel cam for movement thereby upon translation thereof and having a pair of parallel racks disposed to be alternatively engaged with a pinion for rotation of the print element. In the case shifting operation, the rack member is first moved in a first direction, with one of the racks being engaged with the pinion, to rotate the pinion 90 degrees in a direction, and then the other rack is switched to be engaged with the pinion whereafter the rack member is moved back in the opposite direction to rotate a further 90 degrees in the same direction so that the print head is rotated a total of 180 degrees. The selection mechanism has another disadvantage in that additional specific mechanisms such as a no print key mechanism, which apparently makes the carrier more complicated and contributes to an undesirable increase in the weight of the carrier, must be incorporated for a disabling operation of printing means, ribbon mechanisms or letter feed mechanism during a case shifting operation since the printer would otherwise effect unnecessary printing of a certain character, ribbon lift and feed, or letter feed of the carrier relative to the platen.